The present invention concerns a device for discharging webs.
Such a device, as used specifically in connection with a splicing station where web ends are assembled, is known from DE GM 86 15 787. However, this should not be taken as a restriction of the invention to such applications. Since such webs must be handled primarily in uninterrupted processes, such devices include, in addition to an active take-off roll, yet another roll in a waiting station. Once the active take-off roll reaches the end of the wound-up web, the leading end of the roll still in the waiting station must be attached to the end of the web that has just been unwound. This function is performed by a splicing station where the two web ends are joined, by means of adhesive tape, for example. The process can then be resumed, it being understood that the roll that was originally in the waiting station must now be considered the active take-off roll, whereas a fresh roll is transferred to the waiting station.
To keep the web tension of the discharged web within set limits, i.e., to keep it as constant as possible, a compensation system is provided that measures the current web tension. The deflection of the compensation system can be used to generate a closed-loop control signal by means of which the take-off speed is adjusted to produce the desired web tension.
So that splicing can be performed during uninterrupted operation, according to DE-OS 24 24 302 a web accumulator is provided, in which the web passes over two stationary rolls and a gravity-type roll located between them. The gravity-type roll is movably guided in a longitudinal guide transversely to the line connecting the stationary rolls, and can pay out the temporarily stored web material inside this longitudinal guide as soon as it is required by the downstream processing areas.
An essential problem here is to keep the movable roll operating freely. To this end, seals must be provided on the longitudinal guide to protect it from undesirable contamination. However, these seals create additional friction, in addition to which it must be kept in mind that many of the web materials for processing contain carbon black or the like. This promotes contamination of the longitudinal guide. Especially in the case of traction-sensitive webs, such as those that are the respective subjects matter of DE-PS 195 12 963 and DE-GM 85 15 787, every effort must be made to prevent bunching at the longitudinal guide of the web accumulator.
It is, therefore, an object of the present invention to improve the known device for discharging webs in such a way that especially sensitive webs can continue to run unaffected by mass-dynamic effects, even when the process is being interrupted for operational reasons. In the application of the invention to a web discharging device accompanied by a splicing station, during splicing the web must continue to pass through the processing line under a practically unchanged discharging force even when the web end that has just been discharged is being attached to the leading end of the fresh web.
The invention allows of all embodiments in which the web present in the web accumulator is paid out into the processing line or is fed as needed from the processing line back into the web accumulator for further storage.
In its specific use in conjunction with a splicing station, the invention allows of all embodiments in which the end of the web that has just been discharged confronts the beginning of the newly added web, irrespective of whether the web ends are assembled at the same speed, at a lower speed, or at a speed of 0.
Of practical importance, however, is an embodiment in which the web ends remain stationary for such time as they are being assembled. The capacity of the web accumulator is advantageously tailored to this type of application. Such an application calls for the largest storage capacity, in practice, and should have the simplest construction in order to facilitate the practical implementation of the device according to the invention.
The invention furnishes the advantage that the web accumulator, in order to pay out the web stored therein, is imparted a speed that is set by the open-loop control signal for the controllable drive.
The web pay-out speed can therefore be preset not only over a broad range, but also, by closed-loop control, within a very narrow set of limit values.
This advantage is achieved by the fact that the web accumulator is imparted a web pay-out speed that is so high that when splicing is being performed, the arresting of the web ends that occurs upstream of the web accumulator can go xe2x80x9cunnoticedxe2x80x9d in the processing line downstream of the web accumulator, or, to put it another way, in stop-and-go mode there is always an adequate stored length available to permit vigorous take-off motions at web travel speeds of 60 n/min or more with sharp acceleration and braking gradients, even in the case of traction-sensitive web materials.
Thus, through adjustment of the assigned process parameters and manipulated variables, the entire process can be controlled in a practically transitionless manner during stop-and-go operation or in making the changeover between two web take-off rolls. The forced control of the web accumulator, especially its rate of acceleration as it is being started from neutral position, can be imposed within further limits by means of motors. Thus, even in the case of highly sensitive adhesive tapes made of traction-sensitive materials, the elastic deformability of the material is utilized as needed, during splicing, for example.
Plastic destruction of the web materials can be reliably prevented by the selection of suitable drives with high starting torques/starting accelerations.
The web accumulator can thus be used, for example, for the temporary storage of a length of web equal to the amount consumed, when splicing is taking place, during the periods of stoppage of the web ends in the downstream processing line.
In the case of stop-and-go operation, the dynamic components of demand in the processing line are controlled on a closed-loop basis via the compensation system, which constitutes a position feedback transmitter, and are serviced from the web accumulator within the scope of this closed-loop control.
The web accumulator can also operate bidirectionally, provided that it has enough room for reverse accumulation.
In the case of stop-and-go operation, the active take-off roll replenishes the web accumulator. Provided for this purpose is a closed-loop control system that can be implemented as a simple on-off control system (=two-point control), a three-point control system, or a continuous control system.
Depending on the type of control system selected, the refilling of the accumulator then takes place either in a pumping motion, as in the case of two-point control, for example, or in position-controlled fashion based on the current position of the accumulator, as with continuous control.
In the case of three-point control, the filling of the accumulator proceeds at a preset accumulation speed until the end position, xe2x80x9cShut Off,xe2x80x9d is reached.
The preferred solution is position control combined with the xe2x80x9cShut Offxe2x80x9d end position, in which case continuous control of the delivery speeds of the active take-off roll is performed within a relatively short end-of-travel region before attainment of the xe2x80x9cShut Offxe2x80x9d end position. This method advantageously eliminates the pumping effect that is evident in the case of a two-point control system.
To this end, the movable accumulator roll must be provided with an electronic position interrogator by means of which the position signal is stored in the appropriate control circuit.
When the invention is used in combination with a splicing station, after the web ends have been spliced the controllable drive of the web accumulator, which has been acting in the downstream direction to pay out the web, is either switched off or put in reverse so that the web accumulator can begin filling again to be ready for the next splicing operation.
The open-loop control signal for the web accumulator drive can be a simple on/off signal. The drive is set in motion by the xe2x80x9conxe2x80x9d signal and remains in this state until the xe2x80x9coffxe2x80x9d signal follows. Alternatively, the open-loop control signal can represent the predetermined target value. In this way, when splicing is being performed, the web tension in the downstream processing line can be kept at least within the order of magnitude of the predetermined target value.
Still more exact adherence to the predetermined target value is achieved by means of a reference signal derived from the output signal of the compensation system and the predetermined target value. In this fashion, during splicing the web tension in the processing line can also be adjusted to a value slightly above the predetermined target value. This measure reliably eliminates the risk of formation of folds, since there is no possibility of local slackness in the discharged web.
It is nevertheless essential for this improvement of the invention that, in any case, the web accumulator drive be incorporated into a closed control loop in which the current web tension is measured downstream of the splicing station and is adjusted to a preset value.
The output signal from the compensation system is especially well suited to this purpose, since it is already being adjusted to a value within the order of magnitude of the predetermined target value that prevails during the discharge of the web from the active take-off roll. Especially in the case of traction-sensitive long goods, it is also expedient to use the output signal from the compensation system during regular operation to control a roll drive motor, since such take-off rolls are naturally very heavy and their inertia is therefore high. Thus, causing the active take-off roll to be acted upon by a controlled roll drive motor as a function of the web tension reliably prevents plastic material deformation caused by rapid changes in take-off speed.
This improvement of the invention merits particular attention, therefore, since the output signal from the compensation system can also be used to control the web accumulator drive during splicing. When splicing is to be done, the end of the leading web is reached. The take-off roll concerned no longer needs a controlled drive. Nevertheless, the compensation system adjusts the take-off tension to the predetermined target value, while at the same time, the need arises for the temporarily stored web to be fed into the processing line. Now the output signal from the compensation system can therefore be used to control the web accumulator drive, while the compensation system is simultaneously kept to its predetermined nominal position, thereby maintaining conformity with the predetermined target value.
In this case, therefore, the speed at which web is being paid out of the accumulator must be adjusted in a way that preserves the position of the compensation system.
This purpose is served by the output signal from the compensation system, which naturally changes whenever the compensation system deviates from the prescribed position.
This change in position is ultimately transformed into a corrective signal for the web accumulator drive, by means of which the compensation system is brought back into its nominal position.
The compensation system thus uses its own output signal to control its predetermined nominal position so as to maintain an at least substantially constant tensile load in the downstream portion of the processing line, even during splicing. Normal processing operation then resumes smoothly. In addition, by mere commutation, the output signal from the compensation system can be used to control the web tension both in regular processing mode and during splicing.
If the compensation system is desired to be used as the sole closed-loop control system during normal processing operation, one option is to leave the web accumulator idling in a defined end position during normal processing operation. In this case, fluctuations in web tension are detected only by the compensation system. Such fluctuations do not result in any change in the position of the web accumulator.
In addition to the drives for the splicing station, the drive for the web accumulator must also be able to react very quickly and, in particular, to furnish high rates of acceleration. Electrical or pneumatic drives are therefore candidates for this function. The use of hydraulic drives is also conceivable, although they do entail problems with regard to environmental pollution.
Advantageous improvements of the invention will emerge from the dependent claims.